Coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging. It is applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from the tissue, in vivo, ex vivo or in vitro, and a computer produces images of cross sections of the tissue by using information on how the waves are changed upon reflection.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. More recently, Fourier domain OCT (FD-OCT) techniques have been developed. One example uses a wavelength swept source and a single detector; it is sometimes referred to as time-encoded FD-OCT (TEFD-OCT) or swept source OCT. Another example uses a broadband source and spectrally resolving detector system and is sometimes referred to as spectrum-encoded FD-OCT or SEFD-OCT.
In scanning OCT, a light beam is focused onto the sample under test by a probe. Returning light is combined with light from a reference arm to yield an interferogram, providing A-scan or Z axis information. By scanning the sample relative to the probe, linear or two dimensional scans can be used to build up a volumetric image. One specific application involves the scanning of arteries, such as coronary arteries. The probe is inserted to an artery segment of interest using a catheter system. The probe is then rotated and drawn back through the artery to produce a helical scan of the inner vessel wall.
In the Fourier domain OCT, the axial structure of the sample is reconstructed by applying an inverse Fourier transform to the detected fringe signal. Through the scanning process, the axial structure at each point is combined into to generate a three-dimensional image of that sample.
When looking at cross sections through the three-dimensional image, various types of artifacts become evident, typically exhibited by narrow streaks or bands across the image. These streaks are typically caused by small, unwanted but hard to remove, stray reflections in the system. One example of an artifact-causing reflection is the small reflection from a detector window, but many other sources of unwanted reflections are possible. Instead of eliminating these reflection sources directly, in many cases it may be convenient or necessary to remove the artifact from the image through signal processing, i.e. background subtraction.